Shelley was an adherent to the basic tenet of empiricism, that ‘the senses are the only inlets of knowledge’. Yet he also affirmed that there are things we only ‘feel’ to be true. Rooted in Hume’s distinction between ‘impressions’ and ‘ideas’ – between sensory perceptions and the pictures in our minds, distinguishable only by the relative strength of their appearances – Shelley developed the notion of an ‘inward sense’ that guides us in our feelings or intuitions and discerns between real and ideal things. Above and beyond the philosophy of the British empiricists and the scepticism of Hume, yet rooted in their works, Shelley also developed in his verse a notion of what it would mean for an ‘idea’ to outstrip an ‘impression’ – for the world of the imagination to surpass the real thing, and for poetry to offer up ideas of greater force than empirical reality.